1. Field of the Invention
The present invention relates to a method and an apparatus for evaluating the quality of a semiconductor substrate represented by a silicon wafer such as an epitaxial wafer or the like, namely, for quantitatively evaluating impurities, defects and the like existing in a semiconductor substrate.
2. Description of the Related Art
Up to now, as an evaluation method of this kind, there has been disclosed a method for evaluating an epitaxial wafer used for a light emitting device, which method irradiates an epitaxial wafer for a light emitting device being a compound semiconductor with an excitation light, detects a photoluminescence light generated by excitation of carriers in an active layer of this wafer, and derives a non-radiative life time from a speed of variation in intensity of a photoluminescence light when the speed of variation in intensity of the photoluminescence light comes to be a fixed value (Japanese Patent Laid-Open Publication No. 2000-101,145).
In a method for evaluating an epitaxial wafer used for a light emitting device, composed in such a manner, since a non-radiative life time is a physical property value independent of an excited-carrier density, a good correlation with a luminous efficiency is kept with respect to a high-brightness LED having a high excited-carrier density. As the result, since it is possible to accurately and easily measure a non-radiative life time in an active layer without depending upon an excited-carrier density, it is possible to surely select an epitaxial wafer having a high luminous efficiency and improve the yield rate of manufacturing epitaxial wafers.
And there has been disclosed a method for evaluating a semiconductor device by measuring the decay time constant of a photoluminescence light on the basis of the photoluminescence light obtained by irradiating a semiconductor layer with a pulse light as applying a bias voltage in the forward direction between a p-type clad layer and an n-type clad layer, said semiconductor layer having a smaller band gap than the p-type clad layer and the n-type clad layer and being interposed between the p-type clad layer and the n-type clad layer (Japanese Patent Laid-Open Publication No. Hei 10-135,291 (1998/135,291)). This evaluation method computes the decay time constant of said photoluminescence light by subtracting the intensity of luminescence obtained by applying a bias voltage without irradiating an excitation light from the intensity of said photoluminescence light.
A semiconductor device evaluating method composed in such a manner is suitable for a semiconductor device having a pn junction, particularly a light emitting device such as an LED, a compound semiconductor laser and the like, and obtains the decay time constant of a photoluminescence light of a light emitting device by subtracting the intensity of a photoluminescence light reduced in influence of the inclination of an energy band of a semiconductor layer by applying a bias voltage in the forward direction to a pn junction without being irradiated with an excitation light from the intensity of a photoluminescence light when being irradiated with an excitation light. As the result, since even when the intensity of excitation varies, the inclination of an energy band and the decay time constant are little varied and the decay time constant can be measured more accurately, it is possible to improve the inspection of a light emitting device in accuracy and earlier detect the cause of a defect.
On the other hand, there has been disclosed a method for evaluating the life time of a semiconductor surface, said method evaluating the life time of a semiconductor thin layer or an area near it from the intensity of a light having a specific wavelength generated by recombination of electron-hole pairs generated near the surface of the semiconductor thin layer formed on the main surface of a semiconductor substrate by means of an excitation light having a larger energy than the band gap of the semiconductor to be inspected (Japanese Patent Laid-Open Publication No. Hei 8-139,146 (1996/139,146)); In this life time evaluating method, said light having a specific wavelength emitted by recombination of electron-hole pairs is a band-edge recombination and the area of depth in which electron-hole pairs are generated can be selectively changed by selection of the wavelength of said excitation light. And as its semiconductor substrate, a crystal of 0.1 xcexa9cm or less in resistivity is used in order to make the diffusion length of carriers comparatively short and the intensity of band-edge recombination stronger.
In a method for evaluating the life time of a semiconductor surface composed in such a way, since the area of depth in which electron-hole pairs are generated can be selectively changed by selecting the wavelength of an excitation light, it is possible to selectively evaluate only the life time of a semiconductor thin layer or the life times of both a semiconductor thin layer and a semiconductor substrate.
However, said existing method for evaluating an epitaxial wafer for a light emitting device disclosed in Japanese Patent Laid-Open Publication No. 2000-101,145 has a disadvantage that although a light emitting device is conceived to emit light in the irradiation domain of an excitation light due to a sufficiently short life time in the order of nanoseconds of an epitaxial wafer (compound semiconductor) used in the light emitting device, in a semiconductor substrate such as an indirect band gap silicon substrate or the like having a long life time in the order of microseconds, an accurate life time cannot be measured without considering the diffusion of carriers excited in a semiconductor substrate by irradiating it with an excitation light.
And said existing method for evaluating a semiconductor device disclosed in Japanese Patent Laid-Open Publication No.Hei 10-135,291 (1998/135,291) has a problem that since an object of measurement is a compound semiconductor having a double hetero-structure having a short decay time constant, although the decay time constant can be obtained with a comparative accuracy by applying a bias voltage in the forward direction to a pn junction and thereby reducing the influence of the inclination of an energy band of a semiconductor layer, in a semiconductor substrate such as an indirect band gap silicon substrate or the like having a long life time a decay time constant cannot be accurately measured without considering the diffusion of carriers excited in the semiconductor substrate.
Further, said existing method for evaluating the life time of a semiconductor surface disclosed in Japanese Patent Laid-Open Publication No.Hei 8-139,146(1996/139,146) has a disadvantage that a domain of depth in which electron-hole pairs are generated cannot be controlled even by changing the wavelength of an excitation light.
That is to say, electron-hole pairs generated in a semiconductor thin layer by being irradiated with an excitation light have a finite life time, they sometimes diffuse and recombine and thereby emit light outside the area irradiated with an excitation light. As the result, even when changing the wavelength of an excitation light, only the area irradiated with an excitation light does not necessarily emit light, and the domain of depth in which electron-hole pairs are generated cannot be controlled.
An object of the present invention is to provide a method and an apparatus for evaluating the quality of a semiconductor substrate, which can accurately evaluate impurities, defects and the like in a semiconductor substrate by obtaining quantitatively the life time of a semiconductor substrate having a long life time without breaking and touching the semiconductor substrate.
Another object of the present invention is to provide a method for evaluating the quality of a semiconductor substrate, which can obtain a photoluminescence light intensity having a positive correlation with the life time of a thin film layer or a bulk substrate and can accurately evaluate impurities, defects and the like in a thin film layer or a bulk substrate without breaking and touching the semiconductor substrate.
The inventors have thought that when a semiconductor substrate having a long life time of the order of several tens to several hundreds microseconds like a polished silicon substrate is used in measurement of the life time of a semiconductor substrate by a photoluminescence method, the decay of a photoluminescence light emitted from the semiconductor substrate cannot follow the chopping of an excitation light even if it is an ordinarily used chopping frequency of several tens to several hundreds Hz, and when the chopping frequency of an excitation light is gradually raised from a low frequency to a high frequency, said photoluminescence light is changed from an intermittent luminescence having a large fluctuation range to a luminescence having a small fluctuation range. The inventors have expected that the dependency upon the chopping frequency of a photoluminescence light varies according to the decay time constant of a photoluminescence light of each semiconductor substrate. In other words, the inventors have found that it is possible to obtain the decay time constant of a photoluminescence light on the basis of variation in the chopping frequency of an excitation light when the photoluminescence light changes from an intermittent luminescence to a continuous luminescence. Thereupon, the inventors have considered the transient response of a photoluminescence light and thus have come to attain the present invention of deriving a life time from the decay time constant of a photoluminescence light.
The first aspect of the present invention is a method for evaluating the quality of a semiconductor substrate, which method irradiates intermittently the surface of a semiconductor substrate with an excitation light, converts the intensity of a photoluminescence light emitted by the semiconductor substrate when it is intermittently irradiated with the excitation light into an electric signal, obtains the decay time constant T of the photoluminescence light from variation of the average intensity of the photoluminescence light converted into said electric signal by increasing gradually the chopping frequency of the excitation light, and computes a life time xcfx84 being an indicator of evaluation of the quality of a semiconductor substrate from an expression xe2x80x9cxcfx84=T/Cxe2x80x9d, where C is a constant.
The method for evaluating the quality of a semiconductor substrate according to the first aspect of the present invention can obtain quantitatively the life time xcfx84 of a semiconductor substrate without breaking and touching the semiconductor substrate, and the obtained life time is a value representing quantitatively accurately impurities, defects and the like in the semiconductor substrate. And this quality evaluation method is suitable for obtaining the life time xcfx84 of a semiconductor substrate having a long life time.
The invention according to the second aspect of the present invention is an apparatus for evaluating the quality of a semiconductor substrate comprising;
a laser device for irradiating the surface of a semiconductor substrate with an excitation light,
a first chopper being provided between the laser device and the semiconductor substrate, chopping the excitation light with which the semiconductor substrate is irradiated at a specified frequency,
a second chopper being provided between the first chopper and the semiconductor substrate, being capable of chopping the excitation light at a variable frequency higher than the first chopper,
a monochromator into which a photoluminescence light emitted by the semiconductor substrate when the semiconductor substrate is intermittently irradiated with the excitation light is introduced,
a photodetector for converting the intensity of a photoluminescence light introduced into the monochromator into an electric signal,
a lock-in amplifier for taking in and amplifying an electric signal converted by the photodetector and a pulse signal issued by the first chopper, and
a controller for reading an electric signal and a pulse signal amplified by the lock-in amplifier, and changing the chopping frequency of the excitation light by controlling the second chopper, wherein;
said apparatus obtains the decay time constant T of the photoluminescence light from variation of the average intensity of the photoluminescence light converted into said electric signal when the controller increases gradually the chopping frequency of the excitation light by controlling the second chopper, and computes a life time xcfx84 being an indicator of evaluation of the semiconductor substrate from an expression (1).
xcfx84=T/Cxe2x80x83xe2x80x83(1),
where C is a constant.
The method for evaluating the quality of a semiconductor substrate according to the second aspect of the present invention, like the first aspect of the present invention, can obtain quantitatively the life time xcfx84 of a semiconductor substrate without breaking and touching the semiconductor substrate, and the obtained life time xcfx84 is a value representing quantitatively accurately impurities and defects in the semiconductor substrate. And this quality evaluation apparatus is suitable for obtaining the life time xcfx84 of a semiconductor substrate 11 having a long life time.
And when a coherent excitation light of a laser or the like is incident on the surface of a semiconductor substrate, this excitation light penetrates the substrate to a depth determined by an absorption coefficient corresponding to the excitation wavelength. This absorption coefficient is a value specific to a semiconductor material, and in case of inputting an excitation light emitted from an argon laser of 488 nm in wavelength into an epitaxial wafer composed of a silicon single crystal, the absorption coefficient is in the order of 1000 cmxe2x88x921 (kayser) and the depth of penetration is about 1 xcexcm. And when a semiconductor substrate is irradiated with an excitation light, an electron and a hole excited by this excitation light, if there is not another recombination center, are recombined between a conduction band and a valence band to perform a band-edge recombination, and if there is a non-radiative center, excited carriers,are recombined in the non-radiative center to make weak the band-edge recombination.
For example, when an argon laser light of 488 nm in wavelength is made to be incident on the surface of an epitaxial layer (5 xcexcm in thickness) of a p/p+ epitaxial wafer, since its penetration depth is 1 xcexcm, said laser light stops within the epitaxial layer of 5 xcexcm in thickness. However, since a carrier has a finite life time in the epitaxial layer, the diffusion of carriers occurs and carriers diffuse to the bulk substrate side. As the result, when the life time of a wafer is short, carriers are difficult to penetrate the bulk substrate, and when the life time of a wafer is long, carriers are easy to penetrate the bulk substrate.
On the other hand, in case of evaluating only an epitaxial layer by means of a photoluminescence method, it is necessary to recognize the difference between a bulk substrate and an epitaxial layer. When the surface of a bulk substrate is irradiated with an excitation light, a photoluminescence light is emitted from the almost whole face in the thickness direction of the bulk substrate, but when the surface of an epitaxial layer of a p/p+ epitaxial wafer is irradiated with an excitation light, since carriers diffuse, a photoluminescence light contains not only light emitted from the epitaxial layer (p) but also light emitted from the bulk substrate (p+). Thereupon, the inventors have found a method for separating a photoluminescence light into a light emitted from the bulk substrate and a light emitted from the epitaxial layer in consideration of carrier diffusion and have come to achieved the present invention.
The invention according to the third aspect of the present invention is improvement of a method for evaluating the quality of a semiconductor substrate by intermittently irradiating the surface of a semiconductor substrate composed of a bulk substrate and a thin film layer deposited on this bulk substrate with an excitation light, making the semiconductor substrate emit a photoluminescence light when the semiconductor substrate is intermittently irradiated with the excitation light, and measuring the intensity of the photoluminescence light.
Its composition is characterized by;
obtaining the steady-state diffusion distribution of carriers generated in a thin film layer when it is irradiated with an excitation light by solving a diffusion equation,
deriving an expression (2) for finding signal data [PL] of a photoluminescence-light intensity from said steady-state diffusion distribution of carriers,
measuring the signal data of two kinds of photoluminescence light intensities by irradiating the surface of the semiconductor substrate with two kinds of excitation lights being different in incident intensity, and
multiplying by a specified value the signal data being smaller in intensity out of two kinds of signal data of photoluminescence light intensities and thereafter subtracting the signal data being smaller in intensity from the signal data being larger in intensity, and thereby obtaining the signal data containing a more amount of light emitted from the thin film layer by eliminating the first term of said expression (2), or obtaining the signal data containing a more amount of light emitted from the bulk substrate by eliminating the second term of said expression (2).                                           [            PL            ]                                CB            r                          =                              p            ⁢                          xe2x80x83                        ⁢                          τ              ⁢                              xe2x80x83                            [                              1                -                                                      (                                          1                      -                                                                                                    p                            b                                                    p                                                ⁢                                                                                                            τ                              b                                                        τ                                                                                                                )                                    ⁢                                      xe2x80x83                                    ⁢                                      e                                          -                                              d                                                                              D                            τ                                                                                                                                                          ]                        ⁢                          xe2x80x83                        ⁢            I                    ⁢                      xe2x80x83                    +                                                                      τ                                      3                    /                    2                                                                    2                  ⁢                                      D                                                              [                              1                -                                                      (                                          1                      -                                                                                                    τ                            b                                                    τ                                                                                      )                                    ⁢                                      xe2x80x83                                    ⁢                                      e                                          -                                                                        2                          ⁢                          d                                                                                                      D                            τ                                                                                                                                                          ]                        ⁢                          xe2x80x83                        ⁢                          I              2                                                          (        2        )            
Where, [PL] is signal data of a photoluminescence light intensity, C is a constant, Br is a radiative recombination coefficient, p is a carrier density in a thin film layer, pb is a carrier density in a bulk substrate, xcfx84 is the life time of a carrier in the thin film layer, xcfx84b is the life time of a carrier in the bulk substrate, D is a carrier diffusion coefficient, d is the thickness of the thin film layer, and I is the incident intensity of an excitation light.
The method for evaluating the quality of a semiconductor substrate according to the third aspect of the present invention can estimate the life time xcfx84 of a thin film layer having a positive correlation with a photoluminescence light intensity of the thin film layer, by eliminating the first term from the expression (2), without breaking and touching the semiconductor substrate, and the photoluminescence light intensity of the thin film layer comes to be a value representing accurately impurities and defects in the thin film layer. And this method can estimate the life time xcfx84 of a bulk substrate having a positive correlation with a photoluminescence light intensity of the bulk substrate by eliminating the second term from the expression (2), and the photoluminescence light intensity of the bulk substrate comes to be a value representing accurately impurities and defects in the bulk substrate. Further, this quality evaluation method is suitable for evaluation of the quality of an epitaxial layer of an epitaxial wafer or a bulk substrate.